[PSUBS-MAILIST] power converter

Tue Apr 26 22:45:23 EDT 2016

Hi guys,
Getting it to AC should be more straightforward in terms of being able to
get a standard product, but still needs a pretty big/expensive
inverter.  The DC rectifier could work but again for the currents involved
the capacitors required for a smooth output would have to be huge, and it's
another significant exercise/stuff to go wrong.  Plus it's probably all
quite inefficient (I wouldn't be surprised if you lost 20% of the
electrical energy just through the conversion).

Cheers,
Steve

On Wed, Apr 27, 2016 at 12:18 PM, k6fee via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> Another option is to use an off the shelf 48v to 120v ac inverter then use
> a bridge rectifier to take it back to DC. To get away from a cooling fan
> the heat sink could be mounted to the hull to dissipate the heat and make
> it quiet.
>
> Keith T.
>
>
>
> Sent from my Verizon, Samsung Galaxy smartphone
>
> -------- Original message --------
> From: Stephen Fordyce via Personal_Submersibles <
> personal_submersibles at psubs.org>
> Date: 4/26/16 6:30 PM (GMT-08:00)
> To: Personal Submersibles General Discussion <
> personal_submersibles at psubs.org>
> Subject: Re: [PSUBS-MAILIST] power converter
>
> Hi Hank,
> I've dabbled a little bit in (small!) DC converters and my guess is that a
> converter that will do the current you're after will
> be expensive/large/complex and probably generate a significant amount of
> heat.  Also seems like an obscure set of operating conditions that will be
> hard to find off the shelf.
>
> You've probably thought about it, but splitting the battery bank
> and rewiring part of it to supply 120VDC is probably the easiest low-tech
> solution if you have enough cells to put in series and achieve the higher
> voltage.  You would get some nice redundancy there too if you
> have 2 battery banks controlling different thruster sets.
>
> As an aside, I suspect 48VDC is pretty lethal, but 120VDC even more so.
>
> I just saw Alan's email come through - running it off lower voltage is
> definitely an option, but depends on the sort of motor.  Ie. a brushed
> motor speed is proportional to the voltage applied (so running at 48VDC
> would give you less than half speed) - no idea what the Perry ones would
> be, but being older they may well be brushed.  Brushless motor speed is
> proportional to frequency applied by the controller.  In both cases torque
> is proportional to current which is proportional to heat generated.  So the
> greater the current in your wires, the more heat needs to be dissipated,
> and if it's not then something will burn out - probably the thin layer of
> insulation on the motor windings.  The trouble is dissipating the heat from
> the inner windings (it has to pass through the outer windings, which are
> producing their own heat) - but like Alan said, if you've got it oil
> compensated that will help a lot with heat dissipation.  The thermal
> conductivity of oil is ~6x that of air (and water ~24x air), although the
> heat transfer is not quite as simple as that.
>
> I'm also pretty sure that the motor will have an inherent resistance that
> will be designed into it appropriate to the nominal voltage and current,
> and related to the size of the wires (ie. as Hank pointed out, thinner
> wires for high voltage as less current is required).  So if you apply
> less voltage to the same resistance, it will draw less current - ie. I
> don't think you will be able to get extra current into the motor at lower
> voltage.  Not quite that simple, but you get the idea.
>
> (that was mostly dredged up from memories of a university subject I did on
> electric motors a long time ago, feel free to correct me!)
>
> Cheers,
> Steve
>
> On Wed, Apr 27, 2016 at 10:18 AM, hank pronk via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
>> Thanks" Kieth,
>> I started to that but I have no idea if one system is better than another.
>> Hank
>>
>>
>> On Tuesday, April 26, 2016 6:11 PM, k6fee via Personal_Submersibles <
>> personal_submersibles at psubs.org> wrote:
>>
>>
>> Hank,
>>
>> You want a dc-dc converter, just Google it.
>>
>> Keith T.
>>
>>
>>
>> Sent from my Verizon, Samsung Galaxy smartphone
>>
>> -------- Original message --------
>> From: hank pronk via Personal_Submersibles <
>> personal_submersibles at psubs.org>
>> Date: 4/26/16 4:53 PM (GMT-08:00)
>> To: Personal Submersibles General Discussion <
>> personal_submersibles at psubs.org>
>> Subject: [PSUBS-MAILIST] power converter
>>
>> HELP,,,
>> Turns  out my Perry thrusters are not 36V but 120 Vdc.  I can only manage
>> 48V dc from Gamma's battery bank.  Can I step the voltage up with something.
>> Hank
>>
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